It might not be immediately obvious just how birds could evolve something as complex as the wings needed for flight. But that's just it: flight was basically incidental to the real adaptation. The secret is a little something called flap-running.

If anything, flap-running looks even more ridiculous than it sounds. It refers to when birds run on the ground, flapping their wings but never actually taking off to fly. Researchers from the University of Montana have now studied this behavior in pigeons. They found that flap-running is used by many pigeons when running up steep inclines and even cliffs, which seems like a strange thing to do...after all, why don't the birds just fly up?

The answer, the researchers say, is that it's much, much more energy-efficient to just flap-run. The birds who flap-run use as little as 10% of the energy they would otherwise need to fly over an incline. In fact, when the researchers placed sensors on the birds to measure muscle activity during flap-running, they could find barely any evidence of exertion at all. Flap-running is an incredibly efficient form of movement for these birds.

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And that, says the researchers, could help explain the evolution of flight. The original birds likely evolved from the tiny dinosaur archaeopteryx, whose forelimbs would never have been strong or aerodynamic enough to achieve flight. But these first birds would have had the biological equipment to take advantage of flight-running.

Natural selection would have favored those who were most proficient at this energy-saving maneuver. The best wings for flap-running would also be the best suited to flight, and so flap-running would have been a crucial part of birds' evolutionary journey towards flight.